Currently, the CIS based thin-film solar cell is widely and practically used. When manufacturing the CIS based solar cell, it is assumed that the thin-film solar cell having high conversion efficiency may be obtained by allowing a cadmium sulfide (CdS) layer to grow as a high-resistance buffer layer on a light absorbing layer formed of a CuInSe2 thin film. Patent Document 1 discloses that, in a chemical bath deposition method for chemically forming a CdS thin film from solution, by immersing the CuInSe2 thin-film light absorbing layer in the solution, high-quality heterojunction with the thin-film light absorbing layer is formed and there is an effect to increase shunt resistance.
Also, Patent Document 2 discloses the manufacturing method capable of obtaining the thin-film solar cell having high conversion efficiency equivalent to that obtained when the CdS layer is used as the buffer layer, by using zinc mixed crystal compound containing oxygen, sulfur and hydroxyl, that is, Zn(O, S, OH)x chemically grown from the solution on the p-type light absorbing layer as the high-resistance buffer layer.
However, in the conventional invention disclosed in Patent Document 1, although the effort has been made to minimize wastes containing Cd having high toxicity when allowing the cadmium sulfide (CdS) layer to grow as the high-resistance buffer layer, since solid CdS and alkaline wastes are generated in large volume, there has been a problem that a waste treatment cost becomes higher and a manufacturing cost of the CIS solar cell becomes higher.
Also, Patent Document 2 discloses the manufacturing method effective for eliminating the CdS buffer layer, which is considered to be essential in the method for manufacturing the thin-film solar cell having high conversion efficiency. In this case also, mixture of solid ZnO/ZnS and alkaline wastes are generated in large volume, so that there has been a problem that the waste treatment cost makes the manufacturing cost of the solar cell high.
Also, since the chemical bath deposition method is nonequilibrium chemical reaction, there has been a problem that film-formation by stable reaction at a certain point may not be performed.
Also, there has been a problem in production that the more the production scale increases, the larger the waste plant required for detoxifying ammonia used for maintaining alkaline property of the solution.
Also, since in-line manufacturing is not possible in the chemical bath deposition method, there has been a problem that production efficiency deteriorates.
In view of such circumstances, it has been earnestly desired to consistently carry out the film-formation of the back electrode layer to the film-formation of the window layer (transparent conductive film) by a dry process instead of a wet process such as the solution growth method when forming the high-resistance buffer layer.
Particularly, since an enormous cost is required in the treatment of wastes containing Cd having high toxicity, a Cd-free film-forming method has been earnestly desired.
    Patent Document 1: U.S. Pat. No. 4,611,091    Patent Document 2: Japanese Patent No. 3249342    Patent Document 3: Japanese Patent Application Laid-Open No. 2006-332440